Decanting and filling apparatus

ABSTRACT

An apparatus for decanting liquid from, and introducing liquid to a vessel such as a centrifuge tube has an upright hollow probe or pipet which is connected to a vertical drive and can ride up and down with its lower end in a centrifuge tube held in a clamp on the apparatus. This lower end is provided with a pair of fluid-sensing contacts connected to a control circuit having a selector switch for &#39;&#39;&#39;&#39;raise&#39;&#39;&#39;&#39; and &#39;&#39;&#39;&#39;lower&#39;&#39;&#39;&#39; modes. Liquid can be passed through the probe in either direction by a continuously operable pump. In the &#39;&#39;&#39;&#39;down&#39;&#39;&#39;&#39; or &#39;&#39;&#39;&#39;lower&#39;&#39;&#39;&#39; position of the selector switch the lower end of the probe automatically follows the falling level, as the liquid is pumped out. In the &#39;&#39;&#39;&#39;up&#39;&#39;&#39;&#39; or &#39;&#39;&#39;&#39;raise&#39;&#39;&#39;&#39; position of the selector switch the probe rises to follow the liquid level as the liquid is pumped into the centrifuge tube. An override switch is provided for vertical displacement of the probe whether or not the contacts are immersed, in the &#39;&#39;&#39;&#39;up&#39;&#39;&#39;&#39; and &#39;&#39;&#39;&#39;down&#39;&#39;&#39;&#39; positions, respectively.

United States Patent Buchler [54] DECANTINGANDFILLING APPARATUS 72 Inventor: Joseph Buchler, 1327 16th St., Fort Lee, NJ. 07024 t [22] Filed: Feb. 20, 1970 [21] Appl. No.: 13,129

[52] US. Cl. ..210/83, 210/96, 210/97,

23/253. R, 324/61 [51] Int. Cl ..B0ld 21/00, B03d 13/00 [58] Field of Search ..210/83-86, 96,

2,617,302 Massiot ..210/85 X [451 Aug. 8, 1972 2,904,751 9/1959 Parsons ..210/96 UX Primary Examiner-John Adee Attorney-Karl F. Ross [57] ABSTRACT An apparatus for decanting liquid from, and introducing liquid to a vessel such as a centrifuge tube has an upright hollow probe or pipet which is connectedto a vertical drive and can ride up and down with its lower end in a centrifuge tube held in a clamp on the apparatus. This lower end is provided with a pair of fluid-sensing contacts connected to a control circuit having a selector switch for raise" and lower" modes. Liquid can be passed through the probe in either direction by a continuously operable pump. 1n the ?down or lower position of the selector switch the lower end of the probe automatically follows the falling level, as the liquid is pumped out. In the up or raise position of the selector switch the probe rises to follow the liquid level as the liquid is pumped into the centrifuge tube. An override switch is provided for vertical displacement of the probe whether or not the contacts are immersed, in the up and down positions, respectively.

3lClaims,7DrawingFigures PATENTEDAUG 8 I972 SHEET 1 [If 4 FIG.

'- INVENTQR.

'BUCHLER JOSEPH 3 Ckatl i105 ATTORNEY P'A'TENTEDAuc 8:972 v I I 3.682.305 snare um I Froc.

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. JOSEPH BUCHLER I BY I Q3 g 1rI i AT TOR N EY PATENTEHnus 81912 SHEET 3 BF 4 IN VEN TOR JOSEPH BUCHLER BY F l- G42 new ATTORNEY PATENTEDAUG a 1912 INVEAU" UR saw u or 4 JOSEPH BUCHLER ATTORNEY DECANTING AND FILLING APPARATUS FIELD OF THE INVENTION The present invention relates to an apparatus for controlledly decanting liquid from and introducing liquid to a receptacle. More specifically, this invention concerns an apparatus for depositing a linear gradient of density of a liquid into a centrifuge tube from a density-gradient generator or for controlledly decanting a centrifuged gradient for drop collection or monitoring distribution of a material to be assayed in a density gradient.

BACKGROUND OF THE INVENTION In density-gradient work, a liquid medium is prepared with continuously or discontinuously varying density or specific gravity such that, upon introduction of a substance containing components of different density, the several components will be distributed at levels in the liquid corresponding to their specific gravities. Generally a centrifugation is employed to ensure distribution of the material to be separated in the liquid at the appropriate levels. It is possible in this manner to fractionally separate the material and carry out analysis if the several layers, zones or density discs are monitored by an analytical device such as a U-V spectrum monitor. It is also possible to gather the individual fragments or components in separate vessels by passing the continuously withdrawn liquid into a fraction collectorlike device. Density-gradient separation thus consists of two stages, the first involving the formation of a density gradient, generally in a centrifuge tube. The second stage involves the withdrawal of the liquid in the tube continuously or selectively to retrieve the fractions or components or otherwise subject them to analysis.

The above operations are usually carried out manually by simply holding the pipet and lowering or raising it as the tube is emptied or filled. The tip of the pipet must be regularly displaced, depending on the pumping speed, since overly rapid lowering can result in partial or complete jumping of a gradient while overly slow lowering will cause the formation of air bubbles in the pump tubing. When depositing a gradient, mixing will occur unless extreme care is exercised by similarly raising the pipet with its tip just at the fluid-surface level.

OBJECTS OF THE lNVENTION It is, therefore, an object of the present invention to provide an apparatus for carrying out the above-mew tioned task.

Another object is to provide such an apparatus which can be used with a common laboratory pump to automatically decant or fill a receptacle, particularly a centrifuge tube.

A further object is to provide such a device which is simple to set in operation and control and which is fully automatic in operation thereafter.

It is another object of my present invention to provide an apparatus for the formation of a density gradient in a centrifuged tube and for removal of the density-gradient liquid and components of a separated substance layered therein, or selective removal of such layers, which affords greater precision than earlier methods and enables precise evaluation of the densitygradient distribution.

SUMMARY OF THE INVENTION The above objects are attained, in accordance with the present invention, in an apparatus having a vertically displaceable hollow probe connected by a conduit to a continuously operable pump, the probe and the conduit forming a continuous tube. The probe is upright and its lower end is adapted to move up and down in a centrifuge tube. A motor is connected to this probe to move it either up or down, depending on the operation to be carried out. The lower end of the tube is provided with a pair of contacts forming a sensor which is responsive to immersion of the end in a fluid. An electronic control circuit between the motor and the sensor causes the top of the tube to follow the vertical displacement of the surface level of the fluid.

In general terms, the method of the present invention comprises the formation of a density gradient in a centrifuge tube or other receptacle by feeding from a density-gradient generator liquid of continuously varying density substantially continuously into a lower portion of the tube via a pipet which is gradually withdrawn upwardly to follow the influx of liquid and thereby form the density gradient without mixing thereof. The substance to be separated is thereafter introduced into the tube and centrifugation carried out to distribute the components or fractions of the sample in layers within the density-gradient liquid corresponding to the density or specific gravity of the liquid at these levels. Thereafter the pipet is again used by feeding it progressively downwardly to follow the liquid level while withdrawing continuously the liquid through the pipet to entrain the successive fractions or components therewith. In general terms, moreover the apparatus according to the invention comprises a sensing means having a pair of AC-energized liquid-responsive electrodes at the end of a pipet, connected via an AC amplifier and rectifying or filtering means to a Schmitt trigger or other discriminator responsive to the presence of a liquid between the electrodes for controlling a reversible motor which raises or lowers the pipet to layer the density gradient or remove the latter.

According to another feature of the invention, a selector switch is provided which is connected to the control circuit and has an up or raise, a down or lower, and a standbyy position. In the up position the probe is raised only on immersion of its lower end in the fluid so that it remains just at the surface level. In the down, position the probe is vertically downwardly displaced only when the contacts are not immersed, once again to hold the probe tip just at or under the liquid level. The standby position stops all motion of the probe.

In accordance with another feature of the present invention, an override or bypass switch is provided which, in the down" position of the selector switch, effects downward displacement of the probe even with the contacts immersed and, in the up position, effects upward displacement of this probe even when the probe tip is above the surface of the fluid.

The apparatus is provided, according to yet another feature of the invention, with a stage below the probe having a clamp arrangement for holding the vessel to be filled or decanted. 1

The control circuit of the system comprises an oscillator connected to the electrodes of the AC conductivity cell, and a circuit which actuates the motor, the circuit being responsive to the impedance across the contacts, as seen by a change in the level of the AC signal applied to them as these contacts are immersed in a uid.

The apparatus as described above allows the simple decanting or filling of a receptacle with, once the operation is started, fully automatic operation. The top of the probe can easily rest between, say, 0.5 mm and 1 mm below the surface of the fluid at all stages of the decanting or filling, as long as the pumping speed does not exceed a certain practical limit.

DESCRIPTION OF THE DRAWING The above objects, features, and advantages will become apparent from the following description, reference being made to the accompanying drawing, in which:

FIG. 1 is a perspective front view of the apparatus accordingto the present invention;

FIG. 2 is a back view of the apparatus, the case being removed;

FIG. 3 is a top view of the apparatus as shown in FIG.

FIG. 4 is a block diagram of the apparatus according to the present invention;

FIGS. 5 and 6 are enlarged views of details of the present invention; and

FIG. 7 is a schematic diagram of the electronic control circuit of the present invention.

6. SPECIFIC DESCRIPTION As seen in FIGS. 1 3 the apparatus has a housing formed by a bent base plate 11 over which fits a cover 13. The plate 10 has two front feet 11 which can be screwed in and out to level the device, and one nonadjustable rear foot 12.

On the face of the housing 10 is a stage arrangement 15 adapted to hold a receptacle R such as a centrifuge tube; A probe assembly 14 above this rides up and down in a slot 16 in the plate 10. The front panel further has a selector switch 17, an override button 18, and a pilot light 19.

Inside the housing 10 is a motor arrangement 20 which is connected to the probe assembly 14 to move this latter up and down in the slot 16, and an electronic circuit board 21 held between two rails 22 and plugged in at 23. The circuitry of this board 21 is coupled to a sensor 24 on the lower end or tip of the probe assembly. The back panel of the apparatus carries a fuse 25, and a line cord 26 fitted with a three-prong adapter 27 passes through this panel.

The stage 15 consists of a plate 32 which is secured to the bottom of the housing 10 and is formed with a slot 31. A pair of synthetic-resin bodies 30 having facing V-shaped surfaces rides in the slot 31. A compound screw 35 threaded through both of the bodies 30 carries a knob 33 at each end. Rotation of either knob 33 in one direction brings the bodies 30 toward each other to clamp a centrifuge tube R, rotation in the opposite direction moves them apart to release it.

A small spirit-level 36 is provided on the front of the plate 32 so that, by adjusting the feet'll, this plate 32 can be made level or precisely horizontal.

The probe assembly 14 consists basically of a synthetic-resin mounting block 37 mounting a probe 38, which latter carries the sensor 24, as shown in FIGS. 5 and 6.

The probe 38 itself comprises a pair of coaxial stainless steel tubes 39 and 40, the latter being longer than the former, separated by a sleeve 40 of insulating.

material, such as Teflon. A synthetic-resin sleeve 50 is adapted to be held by the fingers for manually lowering and raising the probe 38. The inner tube 40 is fitted on its lower end with a noule 49 having a plurality of laterally opening orifices 49 and the outer tube with a pointed finger 41, the two forming a pair of contacts. The lower end of the finger 41 is just level with the orifices 49'. The tubes 39 and 40 are anchored in the sleeve 50 by respective setscrews 50 and 50' and rings 50a" and 50a respectively connected to two conductors 40" and 40 of a wire 46. The finger 41 is ground to a point at 4la and is unitary integral with the remainder of the outer tube.

The mounting block 37 is formed with a vertical throughgoing hole 34 substantially larger than the probe 38, a setscrew 28 is threaded in this block 37 to impinge and clamp the probe 38 in this hole 34. A second synthetic-resin element 43 is held to the back of the block 37 to allow it to slide up and down in the slot 16. A screw 42 passing through both elements 43 and 3 is loaded by a spring 29 to clamp with its end 44 a chain 45 associated with the up-and-down drive arrangement 20 for the probe. Depression of the screw 42 in direction A releases the chain 45 to allow manual displacement of the assembly 14.

The outer and inner tubes 39 and 40 are separately 4 connected through a short cable 46 to a plug .47 received in a jack 48 on the front panel of the device.

The drive arrangement 20 for the probe simply comprises a motor 51 which operates on line current and has an output shaft speed of approximately 0.5 RPM.

. This motor 51 is mounted on standoffs 52 on the front panel 10 of the apparatus. A ball chain 45 leads from a sprocket on the motor to a pulley 53 mounted r'otatably on a spacer 54 secured to the panel 10. In this manner the chain 45 fonns a loop running parallel to the slot 16, and, as mentioned above, one part of this loop is secured to the mount of the probe 38, so that rotation of the motor 51 will drive the chain 45 and, depending on the rotation sense, raise or lower the probe assembly 14.

As shown in FIG. 4, the circuit board 21 includes an 2000-I-Iz=oscillator 62 which is connected to the probe tip 24 in such a way that on immersion of this conductivity sensor 24 in a fluid, its output decreases markedly. This output is fed through an AC amplifier 62 and a filter/rectifier network 64 into a Schmitt trigger circuit 65 which responds differently to the two signal levels corresponding to an immersed and a non-immersed condition of the sensor 24.

As also shown in FIG. 4 the output of the Schmittcircuit discriminator 65 is applied via an amplifier 66 to i the relay 55 while an override switch 18 is provided to allow up and down movement of the pipet, depending upon the setting of a selector switch 17, independently of the sensing means. The selector switch 17 is connected to the discriminator 65 to establish the u" or raise" mode, or the down or lower mode as will be apparent hereinafter.

FIGS. 2 and 3 show a transformer 58 which is part of the power supply of the circuit 21. Further limit switches 56 and 57 are shown which serve to stop the probe assembly 14 at the end of its travel, whether or not the relay 55 is actuated to energize motor 51.

FIG. 7 shows the control circuit in greater detail. It can be seen that the entire circuit is solid-state, thereby allowing it all to be built on the relatively small circuit assembly 21.

The selector switch 17 has four sections 17a, 17b, 17c, and 17d, and four positions: L OFF, DOWN, STANDBY, and UP. In all but the OFF position power is fed through the first section 17a to the transformer 58 of a power supply 67. The output of this transformer 58 is rectified by a diode crl, filtered, and then stabilized by a Zener diode z.

The oscillator 62 comprises a simple RC network whose capacitor C1 is connected across a neon lamp ne. The output of this oscillator is fed through a capacitor C2 to the jack 48 whence it is connected to the tube 40, the tube 39 being connected to ground. The second section 17b of the switch 17 connects the output of this oscillator 62 and the tube 40 to ground in the OFF and STANDBY positions.

An n-channel FET transistor Q1 senses the impedance between the electrodes 39 and 40, and its output is passed with AC amplification through an NPN transistor O2 to the filter 64. Thus, the transistors Q1 and Q2 and their associated bias resistors constitute the AC amplifier 63, since the amount of AC signal loss at the sensor 24 is proportional to the impedance thereacross.

The AC signal output of the amplifier 63 is fed to a twin-T filter 64' and thence to a peak detector 64" having a diode cr2. In this manner, the output of the detector 64" is a DC signal having a relatively low or a relatively high level, corresponding to an immersed or nonimmersed condition of the sensor 24, respectively.

This DC. output is fed into the discriminator 65, which is a Schmitt trigger basically as described on pages 389-394 of Pulse, Digital, and Switching Waveforms by J. Millman and H. Taub (McGraw-l-Iill: I965). The switch gang 17c, in the DOWN position, connects the output of the second transistor O4 to the base of a transistor Q5 which forms a power-amplifier switch 68 for he relay 55. In the UP position of the switch 17 the collector of the first transistor O3 is connected to the switch 68, so that this transistor Q3 constitutes the amplifier 66 of FIG. 4.

The motor 51 is a D. C. motor having two coils 51' and 51", each having one side connected to one side of the A.C. input. In the DOWN position of the switch 17, the gang 17d connects the other side of the A.C. input to a pole 55' of the relay 55. In the actuated position of the relay 55 this A.C. current is fed to the down coil 51" of this motor 51 thereby rotating the motor 51 in the direction necessary to lower the probe assembly 14.

When the relay 55 is not actuated, it switches the other side of the A.C. input through a diode cr3 and across the up coil 51, to stop the motor 51 immediately. The normally-closed limit switch 57 prevents downward motion of the probe assembly 14 even when the switch 17 is in the DOWN position and the relay 55 is not pulled in. i

The switch gang 17d, in the UP position, connects the other side of the A.C. input to the other pole 55 of the relay 55 which, in the actuated position, connects it to the up coil 51' and, in the unactuated position, connects the A.C. source through a diode 04 to the down coil 51", much as above. In the OFF and STANDBY positions, the switch gang 17d completely disconnects the other side of the A.C. line so that the motor 51 cannot be activated.

Thus, in the DOWN position which is illustrated here, two conditions are possible in the circuit, namely:

As long as the sensor 24 is not in the fluid, so that there is effectively an air dielectric between the electrodes of the sensor, none of the output of the oscillator 62 is bled off. A relatively high DC. signal output is produced by the rectifier filter 64', 64" which causes O3 to conduct. Since the collector of O3 is connected to the base of Q4, and this transistors collector is connected to the base of switch transistor Q5, the transistorQ4 does not conduct, but the transistor Q5 does. The relay 55, therefore, is actuated, and A.C. is fed into the down coil 51" of the motor 51. The probe assembly 14 moves downwardly.

When the electrodes of the sensor 24 are immersed, the signal sensed by the transistor Q1 drops. This lower signal is amplified, rectified, and filtered, and a relatively low DC. signal is fed into the transistor Q3. This signal is not, however, sufficient to cause or maintain conduction across the transistor Q3, so that this transistor does not conduct, and the transistor Q4 does. Conduction of the transistor Q4 feeds a signal through the switch gang into the invertor transistor 05 which then does not conduct so that the relay 55 is not actuated. Rectified DC. is thus fed into the up coil 51', and A.C. input to the down coil 51" is stopped. The probe 38 stops any vertical displacement for as long as the contacts of the sensor 24 are bridged by liquid.

Actuation of the override switch 18 with the switch 17 in the DOWN position will bypass the transistor Q5 and actuate the relay 55 even when the contacts of the sensor 24 are shorted.

With the switch in the UP position, two conditions of the circuit 21 are also possible, to wit:

The relatively high DC. output of the filter 64" corresponding to a nonshorted condition of the contacts 41, 49 is fed into the transistor Q3, causing it to conduct. Since in the UP position of switch section 17c the collector of transistor O3 is connected directly to the switching transistor Q5, this latter will therefore stop conducting to release the relay 55, thereby feeding rectified D.C. into the down coil 51", and cutting off all current supply to the up coil 51. Thus, in the UP position of switch 17, as long as the sensor 24 is not immersed, the probe assembly 14 will be arrested (unless, of course, the switch 18 is activated, as above).

With the probe sensor 24 immersed, the relatively low D.C. input into the transistor Q3 will not suffice to cause it to conduct, so that transistor Q5 will be switched, and the relay 55 will remain or move into the actuated position wherein alternating current is fed into the up coil 51'. It is clear that, when the sensor 24 is immersed with the switch 17 in the UP position, the probe assembly 14 will be moved up until the sensor 24 at its tip is pulled out of the fluid.

OPERATION OF THE APPARATUS As shown in FIG. 4, in order to decant a centrifuge tube the filled tube R is set between the clamps 30 and one of the knobs 33 rotated to hold the tube snugly in place. A flexible conduit or tube 59 preferably of some inert material such as Teflon (tetrafluorethylene) is connected through a pump 60 preferably an adjustable pump of the peristaltic type to a piece of analytical equipment, such as an ultra-violet-transmission monitor 61, or to a fraction collector. The tubing 59 face level. Since the probe extends the distance d of 20 around 0.5mm beyond the tip of the finger 41, this downward displacement will suffice to hold the very end of the probe 38 always at the surface of the fluid.

At the bottom of its travel, the assembly 14 will trip the switch 57 to cut off the motor 51.

Should it be desirable to remove just a few particular gradients, the above procedure is followed, but the override button 18 is pressed when the probe stops at the surface of the fluid. This causes the probe to continue to ride down. Once the uppermost gradient to be withdrawn is reached by the tip of the probe 38, the button 18 is released and the gradient is drawn off. If

I the tip of the probe is thus set at the lower limit of the gradient to be drawn off, the pump only need then be actuated to withdraw this gradient which will drop down as it is pumped out. If the particular gradient must be drawn off from its upper limit, it is necessary to lower the probe until its tip is just at this level, then start the pump and actuate the override button to follow the descending upper gradient level down. Then the next further down gradient of interest is removed by actuating again the button 18 to move the probe 38 down to the necessary level, or the switch 17 is moved to the UP position to lift the probe fully out of the li uid.

To layer a gradient, the empty tube R is placed between the jaws of the clamp 30 as above, and the probe assembly 14 is either manually moved down by pushing in the screw 42, or the switch is moved to DOWN until the lower limit is attained, and then switched to STANDBY. The gradient to be layered is pumped down from a density-gradient generator (US. Pat. No. 3,471,062) through the probe 38 and the switch 17 is moved to the UP position. As the vessel 55 fills, the tip of the probe will move up with the liquid level, and will stop when and if liquid influx stops. When the vessel is sufficiently filled, the pump is stopped and the override button 18 is pressed. ln the UP position, this causes AC. to be applied to the up coil 51 to move the probe assembly 14 up.

When, of course, the device is set to operate in the down mode and the override switch is operated to lower the pipet into the density gradient to a selected 65 layer below the surface, release of the override switch will immobilize the pipet and enable the selected layer to be withdrawn.

Because of the extreme accuracy with which this apparatus functions, it is possible to decant gradients while maintaining excellent separation. Similarly, it is possible to layer gradients virtually without mixing.

I claim:

1. An apparatus for decanting or filling a vessel comprising:

a tube having an end substantially vertically displaceable in the vessel and provided with an opening at said end;

pump means operatively connected to said tube for passing a fluid therethrough and thereby vertically displacing the surface level of the fluid in said vessel;

drive means operatively connected to said tube for substantially vertically displacing said end in said vessel;

sensor means at said end including a contact opposite said opening and at a level thereof responsive to immersion of said end in a fluid; and

control means operatively connected between said drive means and said sensor means for automatically effecting vertical displacement of said opening with said surface level on vertical displacement of same.

2. The apparatus defined in claim 1 wherein said tube is at least partially an upright hollow probe.

3. The apparatus defined in claim 2 wherein said sensor means includes a pair of contacts on said end of said probe. 7

4. The apparatus defined in claim 3, further comprising selector means having an up and a down position and connected to said control means, for, in said down position, actuating said drive means for downward displacement of said probe only when said contacts are free of said fluid, and, in said up position, actuating said drive means for upward displacement of said probe only when said contacts are immersed in said fluid.

' 5. The apparatus defined in claim 4, further comprising override switch means operatively connected to said drive means for, in said up position of said selector means, upwardly displacing said probe when said contacts are immersed in said fluid and, in said down" position of said selector means, downwardly displacing said probe when said contacts are free of said fluid.

6. The apparatus defined in claim 5 wherein said contacts form part of an AC conductivity circuit.

7. The apparatus defined in claim 6 wherein said control means includes:

signal-generator means connected across said contacts;

discriminator means for sensing a difi'erence in the level of said output between a first level corresponding to immersion of said contacts in said fluid and a second level corresponding to a condition of said contacts being out of said fluid; and

relay means connected to said discriminator means,

to said selector means, and to said drive means for actuating said drive means according to the level of said output and according to the position of said selector means.

8. The apparatus defined in claim 7 wherein said signal generator means is an oscillator, said control means further including an AC amplifier between said contacts and said discriminator means.

9. The apparatus defined in claim 8 wherein said control means further includes a DC filter between said amplifier and said discriminator means.

10. The apparatus defined in claim 2, further comprising stage means for holding said vessel relatively immovably below said probe.

11. An apparatus for decanting or filling a vessel comprising:

an upright hollow elongated probe having a lower end vertically displaceable in the vessel;

a conduit attached to the upper end of said probe;

pump means operatively connected to said conduit for passing a fluid through said conduit and through said hollow probe;

a pair of electrical contacts at said lower end of said probe; motor means operatively connected to said probe for raising and lowering said lower end in said vessel;

selector means having a down position and an up position; and

electronic control means connected between said contacts and said motor means and to said selector means for, in said down position of said selector means, stopping lowering of said probe only on immersion of said contacts in said fluid and, in said up position of said selector means, for raising said probe only on immersion of said contacts in a fluid.

12. In the density-gradient separation of a substance into components of different density, the method which comprises forming the density-gradient by introducing a pipet into a receptacle at a relatively low point thereof, feeding a liquid of a continuously varying density substantially continuously into said receptacle through said pipet substantially continuously, sensing the presence of said liquid at the end of said pipet and automatically displacing said pipet upwardly to follow the rise in level of the liquid in said receptacle; introducing said substance into said receptacle and distributing said components in layers therein, wherein the liquid density corresponds to the density of the component; and thereafter removing at least one of said components from said receptacle.

13. The method defined in claim 12 wherein said components are removed from said receptacle by introducing a pipet progressively into said receptacle while controlling the descent of the pipet to follow the level of the liquid within said receptacle and continuously withdrawing liquid from said receptacle through said pipet.

14. The method defined in claim 12 wherein said one of said components is removed from said receptacle by lowering a pipet below the surface of the liquid in said receptacle to a location corresponding to said component, automatically immobilizing said pipet, and drawing liquid from said receptacle through said pipet.

15. An apparatus for decanting or filling a vessel, comprising:

a tube having an end substantially vertically displaceable in the vessel; pump means operatively connected to said tube for passing a fluid therethrough and thereby vertically displacing the surface level of a fluid in said vessel;

drive means operatively connected to said tube for substantially vertically displacing said end in said vessel;

sensor means at said end responsive to immersion of said end in a fluid; and control means operatively connected between said drive means and said sensor means for effecting vertical displacement of said end with said surface level on vertical displacement of same, said tube including a pair of upright rigid conductive coaxial tubes and an insulating sleeve therebetween, the outer tube being shorter than the inner tube, .said inner tube extending beyond said outer tube and being provided with a nozzle having at least one lateral orifice, said outer tubes being formed with a descending finger having a lower end substantially level with said nozzle, said nozzle and said finger constituting said sensor means, said pump means being operatively connected to said one tube for passing fluid through same and through said orifice, said apparatus further including a holdingsleeve assembly surrounding said tubes and provided with screw means engaging same for clamping same relative to each other. 16. A probe for sensing and following a movable fluid level, said prove comprising: an outer rigid conductive tube; an inner rigid conductive tube coaxially inside said outer tube; and a sleeve of insulating material between said tubes and mutually insulating same; said inner tube extending beyond said outer tube and having an uninsulated lower end constituting a first fluid-sensing electrode; said outer tube being formed with a descending pointed finger substantially parallel to and having a tip adjacent said lower end, said tip constituting a second fluid-sensing electrode, said lower end having a late opening facing said finger. 17. The apparatus defined in claim 11, further comprising override switch means operatively connected to said motor and control means for, in said up position of said selector means, upwardly displacing said probe when said contacts are immersed in said fluid and, in said down position of said selector means, downwardly displacing said probe when said contacts are free of said fluid.

18. The apparatus defined in claim 11 wherein said contacts form part of an AC conductivity circuit.

19. The apparatus defined in claim 11 wherein said control means includes:

single-generator means connected across said contacts; discriminator means for sensing a difference in the level of said output between a first level corresponding to immersion of said contacts in said fluid and a second level corresponding to a condition of said contacts being out of said fluid; and relay means connected to said discriminator means, to said selector means, and to said drive means for actuating said drive means according to the level 22. The apparatus defined in claim 11, further comprising stage means for holding said vessel relatively immovably below said probe.

23. The apparatus defined in claim 15 wherein said sensor means is a pair of contacts on said end of said probe and said nozzle and finger respectively.

24. The apparatus defined in claim 23, further comprising selector means having an up and a down position and connected to said control means for, in said down position, actuating said drive means for downward displacement of said tubes only when said contacts are free of said fluid and, in said up position, actuating said drive means for upward displacement of said tubes only when said contacts are immersed in said fluid.

25. The apparatus defined in claim'24, further comprising override switch means operatively connected to said drive means for, in said up position of said selector means, upwardly displacing said tubes when said contacts are immersed in said fluid and, in said down position of said selector means, downwardly displacing said tubes when said contacts are free of said fluid.

26. The apparatus defined in claim 25 wherein said contacts form part of an AC conductivity circuit.

27. The apparatus defined in claim 26 wherein said control means includes:

signal generator means connected across said contacts; discriminator means for sensing a difference in the level of said output between a first level corresponding to immersion of said contacts in said fluid and a second level corresponding to a condition of said contacts being out of said fluid; and

relay means connected to said-discriminator means, to said selector means, and to said drive means for actuating said drive means according to the level of said output and according to the position of said selector means.

28. The apparatus defined in claim 27 wherein said signal generator means is an oscillator, said control means further including an AC amplifier between said contacts and said discriminator means.

29. The apparatus defined in claim 28 wherein said control means further includes a DC filter between said amplifier and said discriminator means.

30. The apparatus defined in claim 15, further comprising stage means for holding said vessel relatively immovably below said tubes.

31. An apparatus for decanting or filling a vessel comprising:

a tube having an end displaceable in the vessel and provided with an opening at said end;

pump means operatively connected to said tube for passing a fluid therethrough and thereby vertically displacing the surface level of the fluid in said vessel;

electric drive means operatively connected to said tube for displacing said end in said vessel;

electric sensor means on said tube responsive to immersion of said end in a fluid; and

electric control means operatively connected between said drive means and said sensor means for automatically effecting displacement of said opening with said surface level on alteration of same to follow said surface level. 

2. The apparatus defined in claim 1 wherein said tube is at least partially an upright hollow probe.
 3. The apparatus defined in claim 2 wherein said sensor means includes a pair of contacts on said end of said probe.
 4. The apparatus defined in claim 3, further comprising selector means having an ''''up'''' and a ''''down'''' position and connected to said control means, for, in said ''''down'''' position, actuating said drive means for downward displacement of said probe only when said contacts are free of said fluid, and, in said ''''up'''' position, actuating said drive means for upward displacement of said probe only when said contacts are immersed in said fluid.
 5. The apparatus defined in claim 4, further comprising override switch means operatively connected to said drive means for, in said ''''up'''' position of said selector means, upwardly displacing said probe when said contacts are immersed in said fluid and, in said ''''down'''' position of said selector means, downwardly displacing said probe when said contacts are free of said fluid.
 6. The apparatus defined in claim 5 wherein said contacts form part of an AC conductivity circuit.
 7. The apparatus defined in claim 6 wherein said control means includes: signal-generator means connected across said contacts; discriminator means for sensing a difference in the level of said output between a first level corresponding to immersion of said contacts in said fluid and a second level corresponding to a condition of said contacts being out of said fluid; and relay means connected to said Discriminator means, to said selector means, and to said drive means for actuating said drive means according to the level of said output and according to the position of said selector means.
 8. The apparatus defined in claim 7 wherein said signal generator means is an oscillator, said control means further including an AC amplifier between said contacts and said discriminator means.
 9. The apparatus defined in claim 8 wherein said control means further includes a DC filter between said amplifier and said discriminator means.
 10. The apparatus defined in claim 2, further comprising stage means for holding said vessel relatively immovably below said probe.
 11. An apparatus for decanting or filling a vessel comprising: an upright hollow elongated probe having a lower end vertically displaceable in the vessel; a conduit attached to the upper end of said probe; pump means operatively connected to said conduit for passing a fluid through said conduit and through said hollow probe; a pair of electrical contacts at said lower end of said probe; motor means operatively connected to said probe for raising and lowering said lower end in said vessel; selector means having a ''''down'''' position and an ''''up'''' position; and electronic control means connected between said contacts and said motor means and to said selector means for, in said ''''down'''' position of said selector means, stopping lowering of said probe only on immersion of said contacts in said fluid and, in said ''''up'''' position of said selector means, for raising said probe only on immersion of said contacts in a fluid.
 12. In the density-gradient separation of a substance into components of different density, the method which comprises forming the density-gradient by introducing a pipet into a receptacle at a relatively low point thereof, feeding a liquid of a continuously varying density substantially continuously into said receptacle through said pipet substantially continuously, sensing the presence of said liquid at the end of said pipet and automatically displacing said pipet upwardly to follow the rise in level of the liquid in said receptacle; introducing said substance into said receptacle and distributing said components in layers therein, wherein the liquid density corresponds to the density of the component; and thereafter removing at least one of said components from said receptacle.
 13. The method defined in claim 12 wherein said components are removed from said receptacle by introducing a pipet progressively into said receptacle while controlling the descent of the pipet to follow the level of the liquid within said receptacle and continuously withdrawing liquid from said receptacle through said pipet.
 14. The method defined in claim 12 wherein said one of said components is removed from said receptacle by lowering a pipet below the surface of the liquid in said receptacle to a location corresponding to said component, automatically immobilizing said pipet, and drawing liquid from said receptacle through said pipet.
 15. An apparatus for decanting or filling a vessel, comprising: a tube having an end substantially vertically dis-placeable in the vessel; pump means operatively connected to said tube for passing a fluid therethrough and thereby vertically displacing the surface level of a fluid in said vessel; drive means operatively connected to said tube for substantially vertically displacing said end in said vessel; sensor means at said end responsive to immersion of said end in a fluid; and control means operatively connected between said drive means and said sensor means for effecting vertical displacement of said end with said surface level on vertical displacement of same, said tube including a pair of upright rigid conductive coaxial tubes and an insulating sleeve therebetween, the outer tube being shorter than the inner tube, said inner tube extending beyond said outer tube and being provided with A nozzle having at least one lateral orifice, said outer tubes being formed with a descending finger having a lower end substantially level with said nozzle, said nozzle and said finger constituting said sensor means, said pump means being operatively connected to said one tube for passing fluid through same and through said orifice, said apparatus further including a holding-sleeve assembly surrounding said tubes and provided with screw means engaging same for clamping same relative to each other.
 16. A probe for sensing and following a movable fluid level, said prove comprising: an outer rigid conductive tube; an inner rigid conductive tube coaxially inside said outer tube; and a sleeve of insulating material between said tubes and mutually insulating same; said inner tube extending beyond said outer tube and having an uninsulated lower end constituting a first fluid-sensing electrode; said outer tube being formed with a descending pointed finger substantially parallel to and having a tip adjacent said lower end, said tip constituting a second fluid-sensing electrode, said lower end having a late opening facing said finger.
 17. The apparatus defined in claim 11, further comprising override switch means operatively connected to said motor and control means for, in said ''''up'''' position of said selector means, upwardly displacing said probe when said contacts are immersed in said fluid and, in said ''''down'''' position of said selector means, downwardly displacing said probe when said contacts are free of said fluid.
 18. The apparatus defined in claim 11 wherein said contacts form part of an AC conductivity circuit.
 19. The apparatus defined in claim 11 wherein said control means includes: single-generator means connected across said contacts; discriminator means for sensing a difference in the level of said output between a first level corresponding to immersion of said contacts in said fluid and a second level corresponding to a condition of said contacts being out of said fluid; and relay means connected to said discriminator means, to said selector means, and to said drive means for actuating said drive means according to the level of said output and according to the position of said selector means.
 20. The apparatus defined in claim 19 wherein said signal generator means is an oscillator, said control means further including an AC amplifier between said contacts and said discriminator means.
 21. The apparatus defined in claim 20 wherein said control means further includes a DC filter between said amplifier and said discriminator means.
 22. The apparatus defined in claim 11, further comprising stage means for holding said vessel relatively immovably below said probe.
 23. The apparatus defined in claim 15 wherein said sensor means is a pair of contacts on said end of said probe and said nozzle and finger respectively.
 24. The apparatus defined in claim 23, further comprising selector means having an ''''up'''' and a ''''down'''' position and connected to said control means for, in said ''''down'''' position, actuating said drive means for downward displacement of said tubes only when said contacts are free of said fluid and, in said ''''up'''' position, actuating said drive means for upward displacement of said tubes only when said contacts are immersed in said fluid.
 25. The apparatus defined in claim 24, further comprising override switch means operatively connected to said drive means for, in said ''''up'''' position of said selector means, upwardly displacing said tubes when said contacts are immersed in said fluid and, in said ''''down'''' position of said selector means, downwardly displacing said tubes when said contacts are free of said fluid.
 26. The apparatus defined in claim 25 wherein said contacts form part of an AC conductivity circuit.
 27. The apparatus defined in claim 26 wherein said control means includes: signal generator means connected across said contacts; discrIminator means for sensing a difference in the level of said output between a first level corresponding to immersion of said contacts in said fluid and a second level corresponding to a condition of said contacts being out of said fluid; and relay means connected to said discriminator means, to said selector means, and to said drive means for actuating said drive means according to the level of said output and according to the position of said selector means.
 28. The apparatus defined in claim 27 wherein said signal generator means is an oscillator, said control means further including an AC amplifier between said contacts and said discriminator means.
 29. The apparatus defined in claim 28 wherein said control means further includes a DC filter between said amplifier and said discriminator means.
 30. The apparatus defined in claim 15, further comprising stage means for holding said vessel relatively immovably below said tubes.
 31. An apparatus for decanting or filling a vessel comprising: a tube having an end displaceable in the vessel and provided with an opening at said end; pump means operatively connected to said tube for passing a fluid therethrough and thereby vertically displacing the surface level of the fluid in said vessel; electric drive means operatively connected to said tube for displacing said end in said vessel; electric sensor means on said tube responsive to immersion of said end in a fluid; and electric control means operatively connected between said drive means and said sensor means for automatically effecting displacement of said opening with said surface level on alteration of same to follow said surface level. 